Advances in absorbent article technology have stimulated the search for absorbent (often superabsorbent) materials with desirable properties such as high absorption, high gel strength and low health risks to wearers. Gels comprising clay nanoparticles and polymer have been identified as a new class of absorbent materials which are suitable for such applications. The clay nanoparticles link the polymer structure, providing strong and elastic materials which can thus contain less (or even no) organic bulk cross-linker which make the gel brittle.
One way to incorporate clay nanoparticles into polymer gels is by adding the clay nanoparticles into a traditional polymerization reaction comprising initiator, a high concentration of bulk cross-linker and monomer(s). In such a material, the nanoparticles are not deliberately bonded to the polymer via chemical bonds, but rather physically trapped in its three-dimensional networked structure. Such materials are not considered “claylinked” within the meaning of the present invention.
An example of this type of material is provided in WO 00/72958, which describes an “MCX” mixture comprising monomer, clay particles, cross-linking agent and mixing fluid. The MCX mixture is polymerized upon exposure to a polymerization initiator and a networked polymer/clay alloy is formed. The clay particles are not strongly bonded to the polymer chains in such materials, but are rather embedded in the polymer matrix. That is, they do not function as cross-linkers. If insufficient organic cross-linking is present, therefore, the clay may separate from the polymer, which can cause problems for the integrity and properties of the material and undesired release of clay nanoparticles.
Similarly, CA 2 381 910 describes water-absorbing polymers having interstitial compounds. The clay (zeolite) is not part of the structure as a cross-linker. The zeolite acts to absorb odour.
An alternative way to incorporate clay nanoparticles into polymer gels is by the formation of cross-linked materials comprising nanoparticles which are crosslinked by polymer, so-called “claylinked gels”. An appropriate monomer is added to an exfoliated dispersion of clay nanoparticles, then polymerised with the appropriate initiator system, forming links between the nanoparticles. A three-dimensional network of clay nanoparticles and polymer is formed, in which any particular clay nanoparticle is linked to at least one other clay nanoparticle by polymer. Such materials are stronger than those in which clay nanoparticles are simply embedded in the polymer network, as the clay nanoparticles are chemically bonded to the polymer. For this reason, leakage of clay nanoparticles from such materials is also minimised.
For example, EP 1 160 286 discloses an organic/inorganic hybrid hydrogel based on polyacrylamides.
Another example is found in Zhu et al., Macromol. Rapid Communications, 2006, 27, 1023-1028, which describes a nanocomposite (NC) gel based on clay-linked polyacrylamide. High tensile strength is obtained.
To date, neutral (uncharged) polymers such as polyacrylamides have been used in the formation of claylinked gels (see EP 1 160 286 and other documents cited above). The reason for this is the difficulties encountered when introducing charged species to an exfoliated dispersion of clay nanoparticles, as will now be explained.
At the nano-scale, forces between nanoparticles such as e.g. static charges or van der Waal's forces become significant, meaning that the behaviour of nanoparticles is often rather different to that of larger particles. Nanoparticular clays often have a surface charge distribution which varies significantly over a small distance. For example, exfoliated laponite is a synthetic, disc-shaped silicate, with a thickness of approximately 1 nm and a diameter of 25 nm. In aqueous dispersions, laponite has a strongly negative charge on its faces and a weakly localized positive charge on its edges. The surface charges on such nanoparticles cause the formation of electrical double layers e.g. of Na+ ions in aqueous solution. The electrical double layers which are formed around each clay nanoparticle (or in certain regions of each nanoparticle) cause the nanoparticles to repel each other in aqueous solution, thus providing dispersions of non-interacting particles which are generally transparent or translucent and which have low viscosity.
Addition of charged water-soluble compounds to dispersions of clay nanoparticles reduces the osmotic pressure holding the Na+ ions away from the nanoparticle surface, so that the electrical double layer becomes thinner. The nanoparticles can therefore come closer to one another, which results in their agglomeration. Agglomeration is clearly observable by eye, as low concentration dispersions of clay nanoparticles are initially transparent but become cloudy and form a precipitate upon the addition of a charged compound. High concentration dispersions of clay nanoparticles form gel-like agglomerates upon the addition of charged water-soluble compounds.
Weian et al., Materials Letters, 2005, 59, 2876-2880 describe how montmorrilonite can be stabilised using a reactive intercalating agent, followed by addition of acrylic acid and polymerization thereof.
Haraguchi et al., Macromolecules, 2005, 38, 3482-3490 discusses the mechanism of forming nanocomposite gels based on poly(N-isopropylacrylamide). Silberberg-Bouhnik et al. J. Polym. Sci. B, Polym. Phys. 1995, 33, 2269-2279 discusses the dependence of the swelling ratio of a polyacrylic acid gel (without clay particles) upon its degree of ionisation. V. Can and O. Okay Designed monomers and polymers, vol. 8, no. 5, 453-462, (2005) describes the formation of physical gels between polyethylene oxide (PEO) chains and laponite particles.
Claylinked gels comprising neutral polymers, such as polyacrylamide, are known, as discussed above. Charged polymers (e.g. polyacrylate polymers) have higher water-absorption capacity. This may be due to repulsion between near-lying charged groups within the polymer, which allows greater expansion. Additionally, osmotic pressure builds up between the inside and outside of the charged gels when exposed to water, which drives the absorption process. It would therefore be desirable if claylinked gels could be synthesised which comprised charged polymers.
However, addition of charged monomers to a dispersion of clay nanoparticles causes agglomeration of the nanoparticles, as discussed above, so claylinked gels comprising charged polymers cannot be synthesized using the methods described for neutral polymers such as polyacrylamide.
The present invention provides specific forms of claylinked gels comprising charged polymers, and a method for producing them which overcome the problems associated with known synthesis routes. In this way, claylinked gels comprising charged polymers can be obtained which could not be produced previously.
A superabsorbent material in the form of a foam or a fibrous network has the advantage that it absorbs liquid not only in the material itself (the walls of the pores, or the fibre structure), but also in the pores of the foam, or the interstices between the fibres. However, foam and fibrous materials made of traditional superabsorbent polymers (e.g. polyacrylic acid/polyacrylate polymers) are usually hard and stiff when dry, not elastic enough and brittle when wet—they tend to fall apart under pressure. For these reasons, superabsorbent materials are usually included in absorbent articles in granular form.
It would therefore be advantageous to design a superabsorbent material which could exist in a form which is soft and elastic, and which maintains its elastic properties in both dry and wet states.